(1) Field of the Invention
The present invention relates to methods for increasing the N-glycosylation site occupancy of a heterologous glycoprotein produced in a recombinant host cell modified according to the present invention and genetically engineered to express the glycoprotein compared to the N-glycosylation site occupancy of the therapeutic glycoprotein produced in a recombinant host cell not modified according to the present invention. In particular, the present invention provides recombinant host cells that overexpress a heterologous single-subunit oligosaccharyltransferase, which in particular embodiments is capable of functionally suppressing the lethal phenotype of a mutation of at least one essential protein of the yeast oligosaccharyltransferase (OTase) complex in the presence of the host cell's endogenous OTase complex and methods to using these host cells to produce heterologous glycoproteins.
(2) Description of Related Art
The ability to produce recombinant human proteins has led to major advances in human health care and remains an active area of drug discovery. Many therapeutic proteins require the posttranslational addition of glycans to specific asparagine residues (N-glycosylation) of the protein to ensure proper structure-function activity and subsequent stability in human serum. For therapeutic use in humans, glycoproteins require human-like N-glycosylation. Mammalian cell lines (e.g., Chinese hamster ovary (CHO) cells, human retinal cells) that can mimic human-like glycoprotein processing have several drawbacks including low protein titers, long fermentation times, heterogeneous products, and continued viral containment. It is therefore desirable to use an expression system that not only produces high protein titers with short fermentation times, but can also produce human-like glycoproteins.
Fungal hosts such as Saccharomyces cerevisiae or methylotrophic yeast such as Pichia pastoris have distinct advantages for therapeutic protein expression, for example, they do not secrete high amounts of endogenous proteins, strong inducible promoters for producing heterologous proteins are available, they can be grown in defined chemical media and without the use of animal sera, and they can produce high titers of recombinant proteins (Cregg et al., FEMS Microbiol. Rev. 24: 45-66 (2000)). However, glycosylated proteins expressed in yeast generally contain additional mannose sugars resulting in “high mannose” glycans. Because these high mannose N-glycans can result in adverse responses when administered to certain individuals, yeast have not generally been used to produce therapeutic glycoproteins intended for human use. However, methods for genetically engineering yeast to produce human-like N-glycans are described in U.S. Pat. Nos. 7,029,872 and 7,449,308 along with methods described in U.S. Published Application Nos. 20040230042, 20050208617, 20040171826, 20050208617, and 20060286637. These methods have been used to construct recombinant yeast that can produce therapeutic glycoproteins that have predominantly human-like complex or hybrid N-glycans thereon instead of yeast type N-glycans.
It has been found that while the genetically engineered yeast can produce glycoproteins that have mammalian- or human-like N-glycans, the occupancy of N-glycan attachment sites on glycoproteins varies widely and is generally lower than the occupancy of these same sites in glycoproteins produced in mammalian cells. This has been observed for various recombinant antibodies produced in Pichia pastoris. However, variability of occupancy of N-glycan attachment sites has also been observed in mammalian cells as well. For example, Gawlitzek et al., Identification of cell culture conditions to control N-glycosylation site-occupancy of recombinant glycoproteins expressed in CHO cells, Biotechnol. Bioengin. 103: 1164-1175 (2009), disclosed that N-glycosylation site occupancy can vary for particular sites for particular glycoproteins produced in CHO cells and that modifications in growth conditions can be made to control occupancy at these sites. International Published Application No. WO 2006107990 discloses a method for improving protein N-glycosylation of eukaryotic cells using the dolichol-linked oligosaccharide synthesis pathway. Control of N-glycosylation site occupancy has been reviewed by Jones et al., Biochim. Biophys. Acta. 1726: 121-137 (2005). However, there still remains a need for methods for increasing N-glycosylation site occupancy of therapeutic proteins produced in recombinant host cells.